The murders were set up by Rachel and Leann (both of whom wore the serial killer costume at different points in time). Their motive was to make a viral snuff film, which they accomplish. Rachel successfully faked her own death and Leann was released from police custody before the police realized that she was one of the killers.
The film opens on a crime scene at an abandoned truck stop in the desert where several people have been butchered and burned. They find two survivors, an aspiring actress named Leann (Torrey DeVitto) and her heavily wounded boyfriend, Tyler. The police also recover a number of videos from video cameras and smart phones. The videos were seemingly damaged by the various fires set by the killer. It is up to a team of video analysts led by Detectives Burquez (Radha Mitchell) and Reese (Stephen Moyer) to analyze the video and determine the identity of the killer. Leann is being held by the police as a witness and Tyler is unconscious in a local hospital.
While watching the video, we learn that Leann and Tyler were on their way to Las Vegas. They are accompanied by Leanne's friend Rachel (Caitlin Stasey), who is Leanne's friend and also a filmmaker that is shooting a documentary about Leanne's career. Also on the bus are a ballerina named Vicki, an aspiring magician named Steven, and a woman mysteriously carrying a bag full of money named Katrina. Their bus driver, Ben, is driving to an isolated town to pick up another passenger. En route, the bus flips over after it drives over barbed wire that had been buried under the dirt road. The group then walk to a nearby abandoned truck stop to try and find a working landline or radio to call for help. Rachel and Vicki record much of the events on their video cameras, and some footage is also recovered from Steven, Leann and Katrina's phones.
The passengers are shown being hunted by a serial killer wearing a welding mask and wielding a welding iron. It becomes apparent the killer wanted the murders recorded. The police initially suspect Katrina and Ben as the killer (both of whom are found to have shady pasts) and end up chasing false leads. Meanwhile, they videos show the killer (1) stab Steven to death, (2) use a welding iron to burn off Vicki's limbs before lighting her on fire; and (3) slit Katrina's throat before cutting off her face. It is hinted that Tyler is attacked, and Ben killed, off-screen. The videos end by showing the killer ultimately hunting down Rachel and lighting her on fire (though the killer is injured in his pursuit of Rachel). During the investigation, the footage the police are reviewing is somehow released to the public and it goes viral.
Leanne is interviewed by Detective Reese but doesn't remember anything and is released from police custody. She does note, however, that Steven was still alive for some time after he had been stabbed. Reese eventually realizes that Steven swallowed his phone's memory card just before dying and the police recover it from his body. The memory card has a video of Tyler wearing the killer's costume (a video Steven shot just before being stabbed). Burquez announces that Tyler is the killer and that he will be held in police custody.
When cataloging the video as evidence, Reese eventually realizes that what they had assumed were defects in the video caused by the fire are actually edits. At that moment, "deleted footage" of the murders are released to the press. The deleted footage shows that the situation was all a set up planned by Rachel and Leanne. They are revealed to be the killer (each wearing the killer's costume at different times). Rachel had faked her own death, the girls knocked out Steven and placed him in the costume and shot false footage of him as the killer (and put the memory card down Steven's throat) and Leanne reunited with Rachel after being released by the police. The movie ends with Leanne and Rachel being huge celebrities and threatening that any of their viewers could be actors in their "sequel."
Thanks Evan B!